The present invention relates generally to turbine-driven air cycle refrigeration systems used to provide pressurized cooling air to aircraft cabins or the like. More specifically, the invention provides a uniquely constructed three wheel cooling turbine in which the fan element is positioned between the compressor and turbine elements on a common drive shaft, and in which the pressurized air which drives the turbine is also used to lubricate the shaft's journal and thrust bearings.
Conventional cooling turbines (also known as "air cycle cooling machines") used to supply pressurized cooling air to aircraft cabins comprise three primary elements--a fan impeller, a turbine wheel and a compressor impeller--each fixedly mounted on a common drive shaft. The fan impeller is mounted at one end of the shaft, while the compressor and turbine elements are mounted on the opposite shaft end. Rotational support for the shaft is typically effected through the use of conventional oil-lubricated bearings supplied from a conventional lubricant reservoir supply system. These bearings are positioned outside of the ram air duct to isolate them from the very high temperature heated ram air being drawn across the fan. Because of this, the fan is mounted in a cantilevered fashion upon the portion of the drive shaft projecting into the duct.
During operation of the machine, compressor bleed air from the aircraft's engine is forced through a first heat exchanger positioned in a ram air duct and then into the inlet of the air cycle machine compressor. The compressed bleed air discharged from the machine's compressor is flowed through a second heat exchanger in the ram air duct (in series with the first heat exchanger), through a conventional dehumidifying system, and then into and through the turbine, thereby rotationally driving the turbine and, via the common drive shaft, the compressor and fan. Finally, the expanded and cooled air discharged from the turbine is flowed into the aircraft cabin as environmental control cooling air.
The fan impeller and a portion of the driveshaft are positioned within the ram air duct downstream from the first and second heat exchangers, while the compressor and turbine sections, and the remainder of the shaft, project outwardly of the duct. As the fan is rotationally driven by the turbine it draws ambient air into the inlet end of the ram air duct, across the heat exchangers and the fan itself, and then discharges the air outwardly through the outlet end of the duct. In this manner the turbine-driven fan provides both an additional load for the turbine and a continuous ambient air flow across the heat exchangers to cool the two bleed air streams being flowed therethrough.
Although generally reliable and fairly simple, the conventional air cycle cooling machine system just described has associated therewith several well-known disadvantages and heretofore unsolved problems. One such problem is that because the fan impeller is "outboard" mounted (i.e., cantilevered at one end of the drive shaft a considerable distance from the compressor and turbine and their housings near the other end of the shaft) its maximum permissible rotational speed dictates the design parameters of the turbine and compressor, thereby artificially limiting their efficiencies.
More specifically, since there is a critical rotational speed above which the cantilevered fan cannot be operated, such critical speed also sets the maximum rotational speed of both the compressor and fan. Thus, for given bleed air flow rates through the compressor and turbine, the sizes and configurations of these elements must be designed so that their rotational speeds do not exceed the maximum permissible fan speed. This typically means that both the compressor and turbine must be operated with considerably less efficiency than would otherwise be possible.
Another problem is that the fan must usually be of a special lightweight material, such as titanium, to withstand the high temperature (typically in excess of 450.degree. F.) downstream from the dual heat exchangers, and to lessen the rotationally-induced bending stress on the shaft (thereby permitting a higher maximum fan speed). The necessity of using a titanium fan impeller, of course, markedly increases the cost of the air cycle machine.
Finally, in conventional air cycle machines it is necessary to isolate their oil-fed bearings from the high temperature air exiting the ram air duct heat exchangers. As previously described, this requires that all of the air cycle machine except the fan and its cantilevered shaft portion be positioned outside of the ram air duct--an orientation which typically results in the air cycle machine projecting laterally a considerable distance from one side of the duct, thereby adding appreciably to the overall equipment space requirements.
Accordingly, it is an object of the present invention to provide an air cycle cooling machine which reduces or eliminates above-mentioned problems and disadvantages associated with conventional air cycle cooling apparatus.